1. Field of the Invention
The present invention relates to a semiconductor memory circuit.
2. Description of the Related Art
A technique of forming a thin film transistor (TFT) by using a semiconductor thin film formed over a substrate with an insulating surface has attracted attention. Thin film transistors have been used in display devices typified by liquid crystal televisions. Silicon-based semiconductor materials such as amorphous silicon and low temperature crystallized silicon are known as materials for semiconductor thin films that can be applied to thin film transistors, and as another material, an oxide semiconductor has also attracted attention.
As a material for the oxide semiconductor, zinc oxide or a material containing zinc oxide as a component are known. Also, a thin film transistor made of an amorphous oxide (an oxide semiconductor) with an electron carrier concentration of lower than 1018/cm3 is disclosed (see Patent Document 1 to Patent Document 3).
Meanwhile, memory devices utilizing thin film transistors have been applied to semiconductor devices with various names such as RFIDs (radio frequency identification), ID tags, IC tags, wireless tags, electronic tags, and wireless chips. Using this semiconductor device, an individual object or product is given an individual identification number (that is, identification: ID), with which the object is identified, cross-checked with history or the like by wireless communication with a wireless readout device in each step or the like, and this identification can be utilized in various fields such as production management, inventory management, and merchandise management. Many of these semiconductor devices each include a circuit using a semiconductor substrate of silicon or the like (referred to as IC (integrated circuit) chip) and an antenna, and the IC chip includes a memory device, a wireless circuit, a control circuit, and the like. Regarding the memory device, the identification number needs to be written in every IC that is to be produced only once during manufacturing, and after that, there is a method of holding information of every step that corresponds to the identification number on a management system side, and a method of writing or rewriting information of every step on an IC chip side, in addition to during manufacturing of the IC chip. For the method of writing only once, there is an example of using an organic memory and an example of using a mask ROM (Read Only Memory) during an exposure step during a semiconductor process. Also, there is an example of utilizing a memory element that utilizes a characteristic that is obtained when a thin film transistor including an oxide semiconductor is irradiated with ultraviolet light.